The invention relates to an apparatus for measuring the alcohol percentage in the breath of an examinee, in particular to establish whether the examinee is fit to drive a motor car.
It is known to measure this alcohol percentage by means of a blow pipe, which contains certain crystals of which the color changes when the alcohol percentage of the breath blown into the blow pipe exceeds a predetermined limit. However, this blow pipe has the disadvantage that the measurement is rather inaccurate.
Furthermore, there has been disclosed a gas indicator containing a gas detector of which the electric resistance changes upon exposure to a gas. The gas detector may comprise a body of a semi-conductive material, in particular a metal oxide of the N-type, such as tin oxide, zinc oxide, or ferric sesquioxide. Two spaced electrodes are incorporated in the body, and one of these electrodes serves as a filament for heating the body. The resistance of the gas detector, which is low in its cold condition, is considerably increased by the heating. When the hot gas detector is exposed to a gas, the resistance decreases again in dependence on the concentration of the gas. The gas detector may be responsive, inter alia, to hydrogen, carbon monoxide, methane, propane or alcohol vapor.
The above-mentioned gas indicator is intended to measure relatively large gas concentrations in an open space. In such a measurement, slight variations of the resistance of the gas detector have no influence on the results. Slight resistance variations of this kind may be due, for instance, to an air current along the detector, causing a slight cooling of the body, whereby the resistance is somewhat decreased.
If a gas indicator of this kind is used to measure the alcohol percentage in the breath of a person by blowing the breath onto the gas detector, the latter is cooled by the air flow, so that the resistance is not only decreased by the alcohol percentage, if any, but also by the cooling. For the small percentages to be measured in such a case (0.05-0.08%), this effect has a considerable influence, so that a reliable measurement by means of the gas indicator is not possible.